There has conventionally been an image forming apparatus which is arranged so that to visualize an electrostatic latent image formed on a photosensitive drum, toner is adhered thereto, and a resultant toner image is transferred onto a transfer material wound around a transfer drum.
As shown in FIG. 19, such an image forming apparatus includes a cylinder 101 having a dielectric layer 101a, in which corona chargers 102 and 104 are separately provided. The corona charger 102 is intended to attract a transfer material P, while the corona charger 104 is intended to transfer a toner image formed on a surface of the photosensitive drum 103 onto the transfer material P, and with these charger 102 and 104, the attraction of the transfer material P and the transfer operation are independently performed.
As shown in FIG. 20, there has also been another image forming apparatus which includes a double-layer cylinder 201 and a grip system 202. The cylinder 201 has an outer semi-conductive layer 201a and an inner base 201b, and the grip system 202 is intended to hold the transfer material P transported thereto along a circumferential surface of the cylinder 202. In this image forming apparatus, an edge of the transfer material P transported to the grip system 202 is caught by the grip system 202 so that the transfer material P is held along the surface of the cylinder 201, and thereafter the toner image formed on the photosensitive drum 103 is transferred onto the transfer material P. In this case, the surface of the cylinder 201 is charged either by applying a voltage to the semi-conductive layer 201a as the outer layer of the cylinder 201 or by causing a charger disposed inside the cylinder 201 to discharge.
In the case of the image forming apparatus shown in FIG. 19, however, the following problem arises: since the cylinder 101 serving as a transfer drum has a single-layer structure including only the dielectric layer 101a, the corona chargers 102 and 104 need to be provided inside the cylinder 101, thereby necessarily limiting the size of the cylinder 101 and making the apparatus bulkier.
The image forming apparatus shown in FIG. 20 needs less chargers, since the cylinder 201 serving as a transfer drum is arranged in the double-layer structure so that the cylinder 201 is charged for transferring the toner image to the transfer material P. The overall structure of this image forming apparatus, however, is complicated since it is equipped with the grip system 202.
Meanwhile, the Japanese Publication for Laid-Open Patent Application No.173435/1993 (Tokukaihei 5-173435) has disclosed an image forming apparatus provided with a transfer drum having a conductive drum base, a resilient layer, and a dielectric layer. The resilient layer is formed on the conductive drum base and is made of a foam material, and the dielectric layer covers the resilient layer. This image forming apparatus has a mechanism with which toner images of respective colors which are sequentially formed on the photosensitive drum are sequentially transferred onto transfer paper attracted to the transfer drum so as to be superimposed on one another, thereby producing a full-color image on the transfer paper.
In this image forming apparatus, an attracting roller as charging means is disposed close to the transfer drum. By applying a voltage to the attracting roller, discharge is caused in a gap between the drum base and the dielectric layer, thereby generating electric charge. As a result, the transfer paper is electrostatically attracted onto the dielectric layer. Here, the foam material serves as a gap keeping material.
The foregoing image forming apparatus, however, has a drawback in the number of power sources. Specifically, at least two power sources are necessitated. One is a power source for causing the attracting roller to attract the transfer paper to the transfer drum, and the other is a power source for applying to the transfer paper a voltage with an opposite polarity of that of the toner so that the toner image is transferred to the transfer paper wound around the transfer drum.
To solve the drawback, the applicant of the present application has proposed an image forming apparatus (for example, the Patent Application No. 295194/1994 (Tokuganhei 6-295194)) having (1) a photosensitive drum (image carrying body) on which a toner image is formed, (2) a transfer drum (transfer means main body) composed of a dielectric layer, a semi-conductive layer, and a conductive layer formed in this order from a surface of contact with the transfer paper, (3) a power source (voltage applying means) for applying a predetermined voltage to the conductive layer, and (4) a ground roller (potential difference producing means) for pressing the transfer paper against the surface of the dielectric layer and for producing a potential difference between the voltage-applied conductive layer and the transfer paper. The ground roller is disposed on an upstream side, in the transfer paper transporting direction, to a position at which the transfer is performed.
In the image forming apparatus, a potential difference is produced between the conductive layer and the transfer paper by applying a voltage to the conductive layer of the transfer drum while pressing the ground roller against the transfer drum with the transfer paper therebetween, and this potential difference causes local discharge in a region (hereinafter referred to as a contact region) where the ground roller is brought into contact with the transfer drum by pressure, thereby causing injection of charge. As a result, charge with a polarity opposite to that of the voltage applied to the conductive layer is induced on the transfer paper and accumulated thereon, whereby the transfer paper is electrostatically attracted to the dielectric layer. Further, the voltage applied to the conductive layer causes the toner image to be transferred onto the transfer paper.
With this arrangement, the voltage required may be lower and the control of voltage is easily performed, since the attraction of and the transfer with respect to the transfer paper are executed by the local discharge occurring in the contact region and the accompanying injection of charge. Besides, the image forming apparatus can be manufactured at a lower cost, since the power source for causing the transfer paper to adhere to the surface of the dielectric layer, that is, the surface of the transfer means, and the power source for causing the toner image formed on the image carrying body to be transferred onto the transfer paper may not be separately formed.
Tokuganhei 6-295194, however, does not explain in detail an arrangement of ends, in a direction of a rotational axis thereof (a direction orthogonal to a rotational direction), of the transfer drum. The applicant has eagerly studied for further perfection of the image forming apparatus of that invention, and as a result it was discovered that under conditions of high temperature and high humidity, the electrostatic attractive force exerted to the transfer material might lower thereby causing imperfect transfer. This is explained as follows: since under the condition of high temperature and high humidity, respective surface electric resistances of the layers constituting the transfer drum lower, the electric charge accumulated on the transfer paper moves through the surface of the dielectric layer and the ends of the transfer drum to the semi-conductive layer, and then, to the conductive layer through a surface of the semi-conductive layer.